The present invention relates to a voltage drop detecting circuit, more particularly, to a voltage drop detecting circuit by which a drop in a voltage being monitored such as a supply voltage can be detected with a more accurate threshold value and which has such a structure as to permit incorporation into ICs.
In circuits having a memory backed up with a battery, it is necessary that the source of power supply to the memory is changed to the backup battery when the supply of power to the whole circuit is interrupted. The timing of this change may be determined by a voltage drop detecting circuit which, while monitoring the supply voltage, generates a voltage drop detection signal when the supply voltage has dropped below the operating voltage of the memory.
FIG. 3 is a block diagram showing a prior art voltage detecting circuit, in which reference numeral 1 designates a comparison circuit, 2 is a reference voltage generation circuit, and 3 is a detection voltage generation circuit.
The comparison circuit 1 is chiefly composed of a comparator; it compares detection voltage A with reference voltage B and outputs a voltage drop detection signal C if the detection voltage A is smaller than the reference voltage B.
The reference voltage generation circuit 2 is typically composed of a diode or a transistor and is basically intended to generate reference voltage B of a given value that is independent of the variation in the supply voltage.
The detection voltage generation circuit 3 is composed of resistors connected in series between a terminal connected to the supply voltage Vcc and a terminal connected to the ground (GND) and it generates detection voltage A by voltage division of the supply voltage Vcc through resistor change. The ratio between the resistances provided by the resistors is specified in such a way that the detection voltage A coincides with the reference voltage B when the supply voltage Vcc is equal to the threshold level to be detected.
Given the circuit structure described above, voltage drop detection signal C will not be produced as output when the supply voltage Vcc is of a steady-state value since the detection voltage A is greater than the reference voltage B.
However, if the supply voltage Vcc has dropped below the threshold level, the detection voltage A becomes less than the reference voltage B, causing voltage drop detection signal C to be produced as output.
Hence, the necessary processing may be performed at a timing equal to that of outputting of voltage detection signal C. To take the above-described case as an example, one may insure that the source of power supply to the memory is changed to the backup battery at a timing equal to that of outputting of voltage detection signal C.
A problem with the prior art voltage detector circuit is that the reference voltage generation circuit which is typically composed of a diode or a transistor experiences variations in the reference voltage from one product IC to another on account of the effects of fabrication process and the materials used. One of the methods that have been proposed to eliminate these variations is to repeat measurements and adjustments by laser trimming or some other suitable technique until the necessary precision is attained for individual ICs.
However, this method requires expensive facilities and a lot of steps, so the scope of its application is limited to areas of high added value such as measuring instrumentation. Hence, the voltage drop detecting circuit in general-purpose ICs of low added value has suffered from the problem that the threshold value for detection varies greatly from one IC to another.